Generally it's a cleaner that helps keep your engine running better by cleaning out some of the deposits left on cylinder walls and piston heads due to combustion.
So I'm going to use it on my Chrysler Grand Voyager, 2.8 CRD engine, it's a big 2.8 engine with an automatic gearbox, so not a very efficient engine, and it's pulling around a big 1.5tonne (ish) car too.
I was sceptical, as I know a little about how these work and didn't see that it would make much difference, but at the same time wanted to give them a try and see how I got on.
(I also have the petrol version which I'll test out on my wifes car soon and show the results too)
So, the instructions are wait until you're down to 1/3 of a tank left, add the Halfords Diesel Emissions Cleaner to the tank then drive for a further 20-30 miles before filling up as normal. No need to drive differently, etc, so pretty straight forward.
So to test how it performed, I have my previous car computer miles to the gallon indicator, which before the cleaner was showing up at 28. This is a big diesel people carrier I drive so it doesn't get great fuel economy so let's see how it does!
After driving with the fuel cleaner in for 25 miles, I then got to refuel, so reset the trip counter
Then started to use the car as normal, I had a few town drives and a couple of longer journeys so used up around half a tank again, and at the end was quite impressed! The fuel economy was showing 33.7 which is quite a move up from 28.
A lot more than I was expecting to be honest! So all in all I think this is a good product, it definitely made a difference in this VERY unscientific test (So don't tell me how wrong/flawed my testing was, I know it wasn't scientific!) and to me any sort of improvement on a big old diesel engine like mine is a good thing.
I'll post a full video up soon on my testing and the results so you can see what you think.
Why not try it yourself http://www.halfords.com/motoring and tell me how you get on, I'd love to hear back and see your results.
Disclaimer: I received this product in exchange for a fair and unbiased review. The review/opinions above are my own review on this product.
EDIT: Also I've recorded a quick video on this test: